Annona Species Monograph.pdf - Crops for the Future

Chapter 8. Genetic Resources
8.3 Ex situ conservation
Ex situ conservation includes various strategies, such as seed storage, in vitro
culture and field genebanking. Annona seeds show an orthodox response to
desiccation and exposure to sub-zero temperatures. Cherimoya seeds tolerate
desiccation to 4.8% moisture content, while soursop seeds tolerate
desiccation to 5% moisture content. The seeds of sugar apple tolerate
desiccation to 1.5% moisture content, and no viability loss occurred during 6
months of hermetic storage at -20°C (Hong et al., 1996). These authors
suggest that Annona seeds can be conserved in conventional seed genebanks
under conditions of 18°C or less, in airtight containers at a seed moisture
content of 5 ± 1%.
In vitro culture techniques can be used for collecting, exchange and ex situ
conservation of species that produce seedless fruits, as well as for
vegetatively propagated plants, including annonas. In vitro storage can be
done by using slow growth techniques, when medium-term preservation is
sufficient, or by cryopreservation in liquid nitrogen at -196°C, if the need is
for long-term preservation. Both techniques present great advantages for
germplasm conservation. In vitro culture also offers the possibility of
eliminating pathogens, and thus conserving and exchanging germplasm
under disease free conditions. Despite their potential, in vitro conservation
techniques are currently used to a limited extent only. This is due principally
to the lack of research to develop protocols for each species (Ashmore,
1997). Although several papers have appeared on annona tissue culture
(Rasai et al., 1995; Lemos and Blake, 1996; Padilla, 1997; Castro et al.,
1999; Encina et al., 1999; Lemos, 2000), a great deal of work remains to be
done on development of methods for in vitro propagation for germplasm
conservation of Annona species.
Emphasis on in vitro research should be placed on conserving specific clonal
material which is well documented. Much of the range of variation can be
conserved using seed storage, and this is more cost-effective than attempting
large in vitro programmes. Despite the availability of seed and in vitro
conservation techniques, in practice the majority of Annona genetic resources
are stored in field genebanks, also called clonal repositories or collections of
living plants, which face higher risks of disease, human error and
environmental hazards than other conservation techniques (Ferreira, 2001).
These collections seem to be mainly breeders' collections, and are rarely
representative of the range of Annona variability that needs to be conserved.
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